Although not specifically defined in the CISG, reasonableness is so defined in the Principles of European
Contract Law. Moreover, the PECL definition of reasonableness also fits the manner in which this concept is used in the CISG. This definition can help researchers apply reasonableness to the CISG provisions in which it is specifically mentioned and as a general principle of the CISG.

As a general principle of the CISG, reasonableness has a strong bearing on the proper interpretation of all
provisions of the CISG. No provision of any law can purport to expressly settle all questions concerning
matters governed by it. The CISG recognizes this and provides in its Article 7(2):

Part One: Such matters are to be settled in conformity with the general principles on which the CISG is based.

[Reasonableness, we submit, is one of the most fundamental principles on which the CISG is based.]

Part Two: In the absence of general principles on which the CISG is based, such matters are to be settled in

conformity with the law applicable by virtue of the rules of private international law.

There is much doctrine in support of the good-faith and uniform-law logic of seeking to apply Part One of
Article 7(2) in lieu of its Part Two, wherever it is reasonable to do so -- see, for example, the scholarly
writings cited with our Annotated text of Article 7. We submit that regarding reasonableness as a fundamental principle of the CISG and reading reasonableness into every article of the CISG, whether specifically mentioned in the article or not, helps tilt the scales in favor of Part One rather than Part Two applications of Article 7(2) -- a tilting of scales that we submit is required by virtue of the good-faith and uniform-law mandate recited in Article 7(1) of the CISG.

"The term 'reasonable person' is an institute of the common law system and after it has been introduced
for the first time into the Hague Uniform Law on International Sales (ULIS), [25] it had provoked stormy
reaction on the part of the lawyers of the civil law system. Professor A. Tunc in his Commentary of Article
9 of the ULIS considers that assessing the conduct of 'a reasonable person' (reasonable man) according to
'the situation proper to the other party' covers both the character and the factual situation that person is in.
[26] Independently of numerous remarks addressed to the UNCITRAL regarding the provision on 'reasonable person' from the ULIS, that standard has remained also in the UN Convention on International
Sale. Undoubtedly, in this respect the most significant is Article 8 by which criteria are determined for the
interpretation of the contract. According to that article, 'statements made by, and other conduct of, a party
are to be interpreted according to his intent where the other party knew or could not have been unaware
what the intent was'. Further text in the same article goes on for the case of an impossibility to apply that
rule while stating that 'statements made by, and other conduct of, a party are to be interpreted according to
the understanding that a reasonable person of the same kind as the other party would have had in the
same circumstances'.

"In contrast to that criterion which is taken over from the common law system, the civil law system
usually speaks of 'a bonus pater familias' or 'good businessman'.[**]

"One should emphasize that it is good that the criterion of 'a reasonable person' is followed by the
sentence according to which his conduct shall be assessed in conformity with the conduct of a person 'of
the same kind'.[27] It was considered that these additional words would make the reasonable person criterion more impartial, since it was related to a person engaged in the same branch of business, or in the same trade, etc.

"Professor Farnsworth considers that the formulation in paragraph 2 of Article 8 is good since it is not
given in an abstract way, instead being specifically related to the conduct of the specific party. This author
further elaborates that a judge shall have regard as to whether the other party is of the same technical
quality, whether he speaks the same language [28] and altogether, whether and to what degree there exists the similarity between the two persons whose conduct has to be compared with one another.

"Additional criteria are formulated in paragraph 2 of Article 8 which are to be used while determining the
conduct (intentions) of the other party and defining 'a reasonable person of the same kind as the other
party'. Circumstances of the case, negotiations, practices and subsequent conduct of the parties are listed
as exempli causa, namely as examples of additional criteria while determining the intents of the
contracting parties which, however, does not mean that a judge or an arbitrator should not take into
consideration some other facts, too, which according to their opinion could be relevant."[29]

References to reasonableness may also be found in arts. 8(2) and (3), 16(2)(b), 18(2), 25, 33(c), 34,
35(1)(b), 37, 38(3), 39(1), 43(1), 44, 47(1), 48(1) and (2), 60(a), 64(2)(b), 72(2), 76(2), 79(4) and 86(1).
For extrapolations of the concept of the "reasonable person" to still other parts of the CISG -- arts. 9 and 74, for example -- see van der Velden [2] who also cites Tesa v. Amram[3] as an illustrative judicial precedent for reasonableness as a general principle of the Convention. The issue before the court was the reasonableness of the length of the period of time set for payment according to article 62, section 2, ULIS. The court stated: "The Uniform Law on International Sale . . . uses in its articles 10, 11, 13, 22, 26(1),
26(4), 37, 42(2), 61(2), 66(2), 74, 88 and 91 the words 'reasonable', 'unreasonable' and 'reasonably';
'reasonableness' is therefore one of the general principles by which, in accordance with Article 17 ULIS,
the questions not expressly settled in the uniform sales law shall be answered. . . ."[4]

Van der Velden interprets the concept of reasonableness as connoting, among other things, "an ethical
standard of behavior so that [one] must . . . be judicious and fair".[5] Maskow calls attention to the fact
that reasonableness is to be defined according to its meaning in socialist, developing and developed
market economy countries, not simply in developed market-economy countries.[6] Bonell also states that
"in applying . . . the 'reasonableness' test in order to determine whether a party in a particular
circumstance has been (re-)acting with due diligence, a judge of a highly industrialized country may not
automatically refer to the standards of care and professional skill normally required from national
business people in domestic affairs. The answer should be found either in the Convention itself or at least
on the basis of standards which are currently adopted also in other legal systems."[7]

Honnold states that "What is 'reasonable' can appropriately be determined by ascertaining what is normal
and acceptable in the relevant trade. This approach is analogous to and is supported by article 9, which
provides that contractual obligations include 'practices established by the parties and usages . . . in the
particular trade'."[8]